Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Taxes

Schedule of effective tax rates differing from U.S. federal statutory rate

	Year Ended December 31,
	2017				2016				2015
	(in thousands, except percentages)
U.S. federal taxes (benefit) at statutory rate	$	(46,206)	(34.00)	%	$	(32,277)	(34.00)	%	$	(24,375)	(34.00)	%
State tax expense		(6,559)	(4.83)	%		(2,842)	(2.99)	%		(2,428)	(3.39)	%
Research and development credits		(1,149)	(0.85)	%		(1,449)	(1.53)	%		(751)	(1.05)	%
Stock-based compensation		(5,036)	(3.71)	%		1,275	1.34	%		1,683	2.35	%
Change in federal tax rate		44,701	32.89	%		—	0.00	%		—	0.00	%
Mark to market fair value adjustments		(390)	(0.29)	%		49	0.05	%		504	0.70	%
Nondeductible settlement for claims		3,873	2.85	%		—	0.00	%		—	0.00	%
Other nondeductible items		1,009	0.74	%		933	0.99	%		841	1.17	%
Change in valuation allowance		10,211	7.51	%		34,453	36.29	%		24,526	34.21	%
Provision for income taxes	$	454	0.31	%	$	142	0.15	%	$	—	—	%

Schedule of tax effects of temporary differences that give rise to significant portions of the deferred tax assets

	December 31,
	2017		2016
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	77,356	$	68,504
Research and development tax credit carryforwards		12,675		8,634
Reserves and accruals		3,404		5,719
Stock-based compensation		2,929		3,220
Total deferred tax assets before valuation allowance		96,364		86,077
Less: valuation allowance		(95,817)		(85,606)
		547		471
Deferred tax liabilities:
Property and equipment		(547)		(471)
Net deferred tax assets	$	—	$	—

Schedule of reconciliation of the beginning and ending amount of gross unrecognized tax benefits

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Balance at beginning of year	$	4,293	$	2,405	$	1,360
Additions based on tax positions related to the current year		1,651		1,836		1,045
Additions for tax positions of prior years		1		52		—
Balance at end of year	$	5,945	$	4,293	$	2,405